My theory, with no facts whatsoever to back it up, has always been that Trump ran for President assuming he'd lose but it would be good publicity for his business enterprises (some have speculated he wanted to launch the Trump News Network afterward). But he kept winning, so he kept running and now he's President.
If you believe this NY Magazine story, that's not far from the truth:
Donald Trump Didn’t Want to Be President--One year ago: the plan to lose, and the administration’s shocked first days
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